Human copper-zinc superoxide dismutase (SOD1) is a 32 kDa homodimeric metalloprotein that catalyzes the conversion of superoxide radical into molecular oxygen and hydrogen peroxide. The enzyme is particularly abundant in red blood cells and spinal tissue. Approximately 114 different single site mutations in human SOD1 have been linked to an inherited form of amyotrophic lateral sclerosis (ALS, Lou Gehrig's disease, motor neuron disease). Both the inherited (FALS) and sporadic (SALS) forms of the disease are characterized by progressive paralysis resulting from motor neuron degeneration and death. It is now established that SOD1-linked FALS results from the gain of a cytotoxic property and not a loss of enzymatic function. Evidence is accumulating that the toxic property comes from the ability of the mutant SOD1 proteins to assemble into higher order structures (soluble oligomers and insoluble aggregates) that somehow interfere with the neuronal cellular machinery. Using the well established tools of single crystal X-ray diffraction, we recently observed that six different metal-deficient FALS mutant SOD1 proteins can form "amyloid-like" fibers that are somewhat reminiscent of the types of fibers seen in other neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. The pathogenic SOD1 proteins are able to self- associate but the normal, unmutated SOD1 proteins cannot. The experiments outlined in this continuing project are designed to probe pathogenic SOD1 structure and to help answer the following questions: 1) What are the structural consequences of the FALS mutations and how do these amino acid substitutions render the molecule toxic? 2) Does the loss of metal ions play a role in FALS SOD1 pathogenicity? 3) How does the presence or absence of the intrasubunit disulfide bond influence the structural and biophysical properties of pathogenic SOD1? 4) Is the mode of self-association we observe in X-ray studies the basis for how the pathogenic SOD1 proteins aggregate in living cells? 5) What are the structural elements of SOD1 proteins that are recognized by the 20 S proteasome so that they may be degraded? 6) Could soluble oligomers (protofibrils) and/or insoluble amyloids of pathogenic SOD1 act as proteasomal inhibitors? Answers to questions such as these are required for a molecular understanding of SOD1 linked FALS and for the design of therapeutic agents aimed inhibiting the aggregation process.